The carp interface is a pseudo-device which
implements and controls the CARP protocol.
carp allows multiple hosts on the same
local network to share a set of IP addresses. Its primary purpose is to ensure
that these addresses are always available, but in some configurations
carp can also provide load balancing
functionality.

A carp interface can be created at runtime
using the ifconfig
carpNcreate command or by setting up a
hostname.if(5)
configuration file for
netstart(8).

To use carp, the administrator needs to
configure at minimum a common virtual host ID (VHID) and virtual host IP
address on each machine which is to take part in the virtual group. Additional
parameters can also be set on a per-interface basis:
advbase and
advskew, which are used to control how
frequently the host sends advertisements when it is the master for a virtual
host, and pass which is used to
authenticate carp advertisements. Finally
carpdev is used to specify which interface
the carp device attaches to. These
configurations can be done using
ifconfig(8), or through the
SIOCSVH ioctl.

carp can also be used in conjunction with
ifstated(8) to respond to
changes in CARP state; however, for most uses this will not be necessary. See
the manual page for
ifstated(8) for more
information.

Additionally, there are a number of global parameters which can be set using
sysctl(8):

net.inet.carp.allow

Accept incoming carp packets. Enabled
by default.

net.inet.carp.preempt

Allow virtual hosts to preempt each other. Disabled by default.

net.inet.carp.log

Make carp log state changes, bad
packets, and other errors. May be a value between 0 and 7 corresponding
with syslog(3) priorities.
The default value is 2, which limits logging to changes in CARP
state.

carp uses IP balancing to load balance
incoming traffic over a group of carp
hosts. IP balancing is not dependent on ARP and therefore works for traffic
that comes over a router. However it requires the traffic that is destined
towards the load balanced IP addresses to be received by all
carp hosts. While this is always the case
when connected to a hub, it has to play some tricks in switched networks,
which will result in a higher network load.

To configure load balancing one has to specify multiple carp nodes using the
carpnodes option. Each node in a load
balancing cluster is represented by at least one
“vhid:advskew”
pair in a comma separated list. carp tries
to distribute the incoming network load over all configured carpnodes. The
following example creates a load balancing group consisting of three nodes,
using vhids 3, 4 and 6:

# ifconfig carp0 carpnodes 3:0,4:0,6:100

The advskew value of the last node is set to 100, so that this node is
designated to the BACKUP state. It will only become MASTER if all nodes with a
lower advskew value have failed. By varying this value throughout the machines
in the cluster it is possible to decide which share of the network load each
node receives. Therefore, all carp interfaces in the cluster are configured
identically, except for a different advskew
value within the carpnodes specification.

IP balancing works by utilizing the network itself to distribute incoming
traffic to all carp nodes in the cluster.
Each packet is filtered on the incoming
carp interface so that only one node in the
cluster accepts the packet. All the other nodes will just silently drop it.
The filtering function uses a hash over the source and destination address of
the IPv4 or IPv6 packet and compares the result against the state of the
carpnode.

IP balancing is activated by setting the balancing mode to
ip. This is the recommended default
setting. In this mode, carp uses a multicast MAC address, so that a switch
sends incoming traffic towards all nodes.

However, there are a few OS and routers that do not accept a multicast MAC
address being mapped to a unicast IP. This can be resolved by using one of the
following unicast options. For scenarios where a hub is used it is not
necessary to use a multicast MAC and it is safe to use the
ip-unicast mode. Manageable switches can
usually be tricked into forwarding unicast traffic to all cluster nodes ports
by configuring them into some sort of monitoring mode. If this is not
possible, using the ip-stealth mode is
another option, which should work on most switches. In this mode
carp never sends packets with its virtual
MAC address as source. Stealth mode prevents a switch from learning the
virtual MAC address, so that it has to flood the traffic to all its ports.
Please note that activating stealth mode on a
carp interface that has already been
running might not work instantly. As a workaround the VHID of the first
carpnode can be changed to a previously unused one, or just wait until the MAC
table entry in the switch times out. Some layer 3 switches do port learning
based on ARP packets. Therefore the stealth mode cannot hide the virtual MAC
address from these kind of devices.

If IP balancing is being used on a firewall, it is recommended to configure the
carpnodes in a symmetrical manner. This is
achieved by simply using the same carpnodes
list on all sides of the firewall. This ensures that packets of one connection
will pass in and out on the same host and are not routed asymmetrically.

In order to set up a load balanced virtual host, it is necessary to configure
one carpnodes entry for each physical host.
In the following example, two physical hosts are configured to provide
balancing and failover for the IP address 192.168.1.10.

First the carp interface on Host A is
configured. The advskew of 100 on the
second carpnode entry means that its advertisements will be sent out slightly
less frequently and will therefore become the designated backup.

If load balancing is used in setups where the carpdev does not share an IP in
the same subnet as carp, it is not possible
to use the IP of the carp interface for
self originated traffic. This is because the return packets are also subject
to load balancing and might end up on any other node in the cluster.

If an IPv6 load balanced carp interface is taken down manually, it will accept
all incoming packets for its address. This will lead to duplicated
packets.